MFC r363907: environ(7): Update the description and include some more environment variables
- Add a better introduction to the DESCRIPTION section
- Add a description for MANPATH and POSIXLY_CORRECT
- Asorted improvements for the usage of some macros
PR: 43823
Submitted by: Lyndon Nerenberg <lyndon at orthanc dot ab dot ca>
Reviewed by: 0mp, bcr
Approved by: 0mp, bcr
Differential Revision: https://reviews.freebsd.org/D25912